Heat Leak Calculator

• Overall U-value method: Q = U × A × |T_in − T_out|
• Single layer conduction: Q = (k × A × |ΔT|) / L and U = k / L
• Multi-layer resistance: R = Σ(L_i/k_i), optional films R_si = 1/h_i, R_so = 1/h_o
• Effective transmittance: U = 1 / (R + R_si + R_so), then Q = U × A × |ΔT|
• Heat flux: q″ = Q / A, and daily energy E = Q × 24 / 1000 in kWh/day

1. Select a method based on your available design data.
2. Enter area and design inside/outside temperatures.
3. For U-value, input the assembly transmittance directly.
4. For single layer, enter material k and thickness L.
5. For multi-layer, choose layer count and enter each L and k.
6. Enable film coefficients when surface effects matter.
7. Press Calculate, then export CSV or PDF if needed.

Heat leak is the steady transfer of thermal energy through an envelope element such as a wall, roof, slab, duct, or tank surface. In construction, it influences comfort, equipment sizing, condensation risk, and operating cost. A practical estimate begins by choosing the right method: use a known U-value for a finished assembly, or build U from material layers when you have conductivity and thickness data.

The governing relationship is Q = U × A × |ΔT|, where Q is heat rate (W), U is overall transmittance (W/m²·K), A is area (m²), and ΔT is inside–outside temperature difference (K). For layer calculations, first compute resistances. Each layer has R = L/k (m²·K/W), using thickness L (m) and conductivity k (W/m·K). Optional surface films are R_si=1/h_i and R_so=1/h_o. Then U = 1/ΣR.

Use the calculator to compare alternatives quickly. Example: a 12 m² wall with U=0.45 and ΔT=15 yields Q≈0.45×12×15=81 W. If conditions persist, daily energy is E = Q×24/1000 ≈ 1.94 kWh/day. Improving the assembly to U=0.30 drops heat leak to about 54 W and 1.30 kWh/day, supporting insulation payback and HVAC load reduction discussions.

Keep inputs consistent and realistic. Conductivity must match the selected units, and thickness should represent the continuous path through the element. For multi-layer walls, include finishes, insulation, and structural layers as appropriate. Remember that thermal bridging (studs, anchors, metal framing) can increase effective heat loss beyond a 1‑D layer stack; when bridging is significant, prefer an effective U-value from standards, manufacturer data, or a detailed model.

For reporting and QA, capture assumptions with every run:

• Method used (U-value, single layer, or multi-layer)
• Area and design temperatures (and the resulting ΔT)
• Layer thicknesses, conductivities, and any film coefficients
• Operating scenario (continuous, intermittent, or seasonal)

Interpret the sign and context of Q carefully. A positive magnitude simply indicates heat flow driven by the temperature gradient; in winter it represents heating load, while in summer it can represent unwanted heat gain that increases cooling demand. For preliminary sizing, engineers often add a margin for wind effects, construction tolerances, and infiltration that are outside the scope of a pure conduction calculation.

The export options help maintain an auditable trail for submittals, value engineering, and troubleshooting. Compare your outputs to the example table on this page to sanity-check magnitudes before sharing results with stakeholders.

What is the difference between U-value and R-value?

R-value is thermal resistance; higher R means better insulation. U-value is overall transmittance; lower U means less heat flow. They are inverses when expressed for the same assembly and units.

When should I use the multi-layer option?

Use multi-layer when you know each layer’s thickness and conductivity, and you want to estimate an overall U-value. It is ideal for conceptual design comparisons before final tested or code-compliant U-values are available.

Do film coefficients really matter?

They can. Interior and exterior surface films add resistance, especially for smooth surfaces and low air movement. If you already have an assembly U-value from a reliable source, leave films off to avoid double-counting.

How do I choose design temperatures?

Use project design conditions from local standards, owner requirements, or HVAC design briefs. The calculator uses the temperature difference, so ensure both temperatures represent the same time basis and operating scenario.

Why might results be lower than measured heat loss?

This tool models conduction through a defined area. Real buildings also lose heat through air leakage, thermal bridges, moisture effects, and edge conditions. Use measured data or detailed models when those factors dominate.

Can I use this for pipes, ducts, or tanks?

Yes, if you approximate the heat-transfer area and use an effective U-value or layer stack. For curved surfaces, compute the equivalent area carefully and consider specialized cylindrical conduction methods for high accuracy.

What outputs are best for reports?

Include the heat rate (W), heat flux (W/m²), and daily energy (kWh/day), plus inputs and assumptions. Export CSV for spreadsheets and PDF for client-ready documentation with consistent formatting.